Field of the Invention
The present invention is related to a display device displaying a virtual image ahead of a visual line of an observer.
Description of the Related Art
Well known is a display device that is called head-up display in which a display unit emitting a driving data such as a running speed, and an optical member such as lens forming the driving data emitted from the display unit at a desired position as a desired-sized display image is embedded in a dashboard of a vehicle, and forms the display image that is overlapped on a scene at the desired position ahead of the vehicle by irradiating the driving data emitted from the display unit to a windshield.
This type of head-up display has better readability than a conventional guidance directly reading instrument due to reduction of visual line movement of a driver.
Disadvantageously, because when running speed of the vehicle increases a focal length of the driver moves far and a visual range becomes narrow, the conventional head-up display posed the following problems upon high speed travelling.
Shortness of a distance to the displaying image requires time to move the focal length of eye from background to the displaying image. Therefore, comparing alternately the background and the displaying image makes the driver (observer) further tired from. Moreover, narrowness of the visual range of the driver increases an occupancy ratio of the displaying image, making the driver feel intrusive.
Japanese Patent Publication No. H5-147456, in order to solve these problems, discloses that the display distance is made larger upon high-speed travelling, and that the number of dots per one character decreases.